![Gray leaf spot Cercospora zeae-maydis 5465607](https://upload.wikimedia.org/wikipedia/commons/1/12/Gray_leaf_spot_Cercospora_zeae-maydis_5465607.png)
Grey leaf spot (GLS) is a foliar fungal disease that affects
maize
Maize ( ; ''Zea mays'' subsp. ''mays'', from es, maíz after tnq, mahiz), also known as corn (North American and Australian English), is a cereal grain first domesticated by indigenous peoples in southern Mexico about 10,000 years ago. Th ...
, also known as corn. GLS is considered one of the most significant yield-limiting diseases of corn worldwide.
There are two fungal pathogens that cause GLS: ''Cercospora zeae-maydis'' and ''
Cercospora zeina
''Cercospora'' is a genus of ascomycete fungi. Most species have no known sexual stage, and when the sexual stage is identified, it is in the genus ''Mycosphaerella''. Most species of this genus cause plant diseases, and form leaf spots. It is a ...
''.
[Ward, J. M. J., E. L. Stromberg, D. C. Nowell, and F.W. Nutter, Jr. 1999. Gray leaf spot: A disease of global importance in maize production. ''Plant Disease'' 83: 884-895] Symptoms seen on corn include leaf lesions, discoloration (
chlorosis
In botany, chlorosis is a condition in which leaves produce insufficient chlorophyll. As chlorophyll is responsible for the green color of leaves, chlorotic leaves are pale, yellow, or yellow-white. The affected plant has little or no ability to ...
), and foliar blight. Distinct symptoms of GLS are rectangular, brown to gray necrotic lesions that run parallel to the leaf, spanning the spaces between the secondary leaf veins.
The fungus survives in the debris of topsoil and infects healthy crops via asexual spores called conidia. Environmental conditions that best suit infection and growth include moist, humid, and warm climates.
Poor airflow, low sunlight, overcrowding, improper soil nutrient and irrigation management, and poor soil drainage can all contribute to the propagation of the disease.
Management techniques include crop resistance,
crop rotation
Crop rotation is the practice of growing a series of different types of crops in the same area across a sequence of growing seasons. It reduces reliance on one set of nutrients, pest and weed pressure, and the probability of developing resistant ...
, residue management, use of
fungicide
Fungicides are biocidal chemical compounds or biological organisms used to kill parasitic fungi or their spores. A fungistatic inhibits their growth. Fungi can cause serious damage in agriculture, resulting in critical losses of yield, quality, ...
s, and
weed control
Weed control is a type of pest control, which attempts to stop or reduce growth of weeds, especially noxious weeds, with the aim of reducing their competition with desired flora and fauna including domesticated plants and livestock, and in natur ...
. The purpose of disease management is to prevent the amount of secondary disease cycles as well as to protect leaf area from damage prior to grain formation. Corn grey leaf spot is an important disease of corn production in the United States, economically significant throughout the Midwest and Mid-Atlantic regions. However, it is also prevalent in Africa, Central America, China, Europe, India, Mexico, the Philippines, northern South America, and Southeast Asia. The teleomorph (sexual phase) of ''Cercospora zeae-maydis'' is assumed to be ''Mycosphaerella'' sp''.''
Host and symptoms
![Conidiophores of corn gray leaf spot fungus](https://upload.wikimedia.org/wikipedia/commons/6/6b/Conidiophores_of_corn_gray_leaf_spot_fungus.jpg)
Corn is the only species that can be affected by ''Cercospora zeae-maydis''. There are two populations of ''Cercospora zeae-maydis'', distinguished by molecular analysis, growth rate, geographic distribution, and
cercosporin
Cercosporin is a red toxin created by the fungal genus ''Cercospora.'' ''Cercospora'' act as pathogens on a variety of plants including corn, tobacco, soybean, and coffee. Cercosporin is a perylenequinone natural product that is photoactivated an ...
toxin production. ''Cercospora zeae-maydis'' differs from its cousin group ''
Cercospora zeina
''Cercospora'' is a genus of ascomycete fungi. Most species have no known sexual stage, and when the sexual stage is identified, it is in the genus ''Mycosphaerella''. Most species of this genus cause plant diseases, and form leaf spots. It is a ...
'' sp. nov. in that it has a faster growth rate in artificial media, the ability to produce the toxin cercosporin, longer
conidiophore
A conidium ( ; ), sometimes termed an asexual chlamydospore or chlamydoconidium (), is an Asexual reproduction, asexual, non-motility, motile spore of a fungus. The word ''conidium'' comes from the Ancient Greek word for dust, ('). They are al ...
s, and broadly
fusiform
Fusiform means having a spindle-like shape that is wide in the middle and tapers at both ends. It is similar to the lemon-shape, but often implies a focal broadening of a structure that continues from one or both ends, such as an aneurysm on a b ...
conidia
A conidium ( ; ), sometimes termed an asexual chlamydospore or chlamydoconidium (), is an asexual, non-motile spore of a fungus. The word ''conidium'' comes from the Ancient Greek word for dust, ('). They are also called mitospores due to the ...
. ''Cercospora zeina'' sp. nov. affects corn in the Eastern Corn Belt and Mid-Atlantic States; ''Cercospora zeae-maydis'' is found in most corn producing areas of western Kentucky, Illinois, Indiana, Iowa, Wisconsin, Missouri, Ohio, and west Tennessee (Midwest). Both populations share the same
symptom
Signs and symptoms are the observed or detectable signs, and experienced symptoms of an illness, injury, or condition. A sign for example may be a higher or lower temperature than normal, raised or lowered blood pressure or an abnormality showin ...
s and
virulence
Virulence is a pathogen's or microorganism's ability to cause damage to a host.
In most, especially in animal systems, virulence refers to the degree of damage caused by a microbe to its host. The pathogenicity of an organism—its ability to ca ...
, the ability of the fungus to invade the host.
Major outbreaks of grey leaf spot occur whenever favorable weather conditions are present (see below). The initial symptoms of grey leaf spot emerge as small, dark, moist spots that are encircled by a thin, yellow radiance (lesions formation). The tissue within the “spot" begins to die as spot size increases into longer, narrower leaf lesions. Although initially brownish and yellow, the characteristic grey color that follows is due to the production of grey fungal spores (conidia) on the lesion surface. These symptoms that are similar in shape, size and discoloration are also prevalent on the corn husks and leaf sheaths. Leaf sheath lesions are not surrounded by a yellow radiance, but rather a brown or dark purple radiance. This dark brown or purple discoloration on leaf sheaths is also characteristic to northern corn leaf blight (''
Exserohilum turcicum
''Setosphaeria turcica'' (anamorph ''Exserohilum turcicum''; formerly known as ''Helminthosporium turcicum'') is the causal agent of northern corn leaf blight in maize. It is a serious fungal disease prevalent in cooler climates and tropical hig ...
''), southern corn leaf blight (''
Bipolaris maydis
''Cochliobolus heterostrophus'' is a fungal plant pathogen. It can cause southern corn leaf blight in maize.
''Cochliobolus heterostrophus'' is found in many tropical regions and in the southern part of the US. ''Cochliobolus'', although not c ...
''), or northern corn leaf spot (''
Bipolaris zeicola''). Corn grey leaf spot mature lesions are easily diagnosed and distinguishable from these other diseases. Mature corn grey leaf spot lesions have a brown, rectangular and vein-limited shape. Secondary and tertiary leaf veins limit the width of the lesion and sometimes individual lesions can combine to blight entire leaves.
Pathogenesis
One reason for the pathogenic success of ''Cercospora zeae-maydis'' is the production of a plant toxin called cercosporin. All members of the genus ''Cercospora'' produce this light-activated toxin during infection.
In the absence of light, cercosporin is inactive, but when light is present, the toxin is converted into its excited triplet state. Activated cercosporin reacts with oxygen molecules, generating active single oxygen radicals. Oxygen radicals react with plant cell lipids, proteins, and nucleic acids, damaging and killing affected cells, and nutrients released during the cell rupture and death feed the ''Cercospora'' fungus.
A study of mutant ''Cercospora'' lacking the gene responsible for cercosporin production demonstrates that, though unnecessary for infection, cercosporin increases the virulence of ''Cercospora'' fungi.
Disease cycle
![Disease Cycle Corn Grey Leaf Spot](https://upload.wikimedia.org/wikipedia/commons/9/9d/Disease_Cycle_Corn_Grey_Leaf_Spot.jpg)
''Cercospora zeae-maydis'' survives only as long as infected corn debris is present; however, it is a poor soil competitor. The debris on the soil surface is a cause for primary inoculation that infects the incoming corn crop for the next season. By late spring, conidia (asexual spores) are produced by ''Cercospora zeae-maydis'' in the debris through wind dispersal or rain. The conidia are disseminated and eventually infect new corn crop.
[Shaner, G. E., Sellers, P. R., Scott, D. H., "Grey Leaf Spot." Department of Botany and Plant Pathology- Purdue University ] In order for the pathogen to actually infect the host, high relative humidity and moisture (dew) on the leaves are necessary for inoculation.
Primary inoculation occurs on lower regions of younger leaves, where conidia germinate across leaf surfaces and penetrate through stomata via a flattened hyphal organ, an appressorium. ''Cercospora zeae-maydis'' is atypical in that its conidia can grow and survive for days before penetration, unlike most spores that need to penetrate within hours to ensure survival. Once infection occurs, the conidia are produced in these lower leaf regions. Assuming favorable weather conditions (see below), these conidia serve as secondary inoculum for upper leaf regions, as well as husks and sheaths (where it can also overwinter and produce conidia the following season). Additionally, wind and heavy rains tend to disperse the conidia during many secondary cycles to other parts of the field causing more secondary cycles of infection. If conditions are unfavorable for inoculation, the pathogen undergoes a state of dormancy during the winter season and reactivates when humid conditions favorable to inoculation return the following season. The fungus overwinters as stromata (mixture of plant tissues and fungal mycelium) in leaf debris, which give rise to conidia causing primary inoculations the following spring and summer.
Environment
Corn grey leaf spot is found by Crous et al. 2006 to flourish under extended periods of high relative humidity
(over two days) and free moisture on leaves due to fog, dew, or light rain. Additionally, heavy rains tend to assist in dispersal of the pathogen. Temperatures between are also required. If the temperature drops below during wet periods or lacks 12 hours of wetness, the extent of disease will be greatly diminished.
In the Midwest and Mid-Atlantic, these conditions are favorable for spore development during the spring and summer months. The infection cycle may take two to four weeks depending on the environment and susceptibility of the corn product used.
Management
In order to best prevent and manage corn grey leaf spot, the overall approach is to reduce the rate of disease growth and expansion. This is done by limiting the amount of secondary disease cycles and protecting leaf area from damage until after corn grain formation. High risks for corn grey leaf spot are divided into eight factors, which require specific management strategies.
factors for grey leaf spot in corn:
# Susceptible hybrid
# Continuous corn
# Late planting date
#
Minimum tillage
Minimum tillage is a soil conservation system like strip-till with the goal of minimum soil manipulation necessary for a successful crop production. It is a tillage method that does not turn the soil over, in contrast to intensive tillage, which ch ...
systems
# Field history of severe disease
# Early disease activity (before tasseling)
# Irrigation
# Favorable weather forecast for disease
There are currently five different management strategies, some of which are more effective than others.
Resistant varieties
The most proficient and economical method to reduce yield losses from corn grey leaf spot is by introducing resistant plant varieties. In places where leaf spot occurs, these crops can ultimately grow and still be resistant to the disease. Although the disease is not eliminated and resistant varieties show disease symptoms, at the end of the growing season, the disease is not as effective in reducing crop yield. SC 407 have been proven to be common corn variety that are resistant to grey leaf spot. If grey leaf spot infection is high, this variety may require fungicide application to achieve full potential. Susceptible varieties should not be planted in previously infected areas (see above).
[Stromberg, E.L. Gray Leaf Spot Disease of Corn
Department of Plant Pathology, Physiology, and Weed Science, Virginia Tech (2009)]
/ref>
Lennon ''et al.'', 2016 uses a near-isogenic line
Inbred strains (also called inbred lines, or rarely for animals linear animals) are individuals of a particular species which are nearly identical to each other in genotype due to long inbreeding. A strain is inbred when it has undergone at least 2 ...
(NIL) to find . Qgls8 is a quantitative trait locus
A quantitative trait locus (QTL) is a locus (section of DNA) that correlates with variation of a quantitative trait in the phenotype of a population of organisms. QTLs are mapped by identifying which molecular markers (such as SNPs or AFLPs) c ...
(QTL) for GLS resistance GLS may refer to:
Science and technology
* GBAS landing system, an aircraft landing system
* General Lighting Service, a type of light bulb
* Generalized least squares, in statistics
* Global location sensor
* Glutaminase, a gene and enzyme
* G ...
originally from the teosinte ''Zea parviglumis
Maize ( ; ''Zea mays'' subsp. ''mays'', from es, maíz after tnq, mahiz), also known as corn (North American and Australian English), is a cereal grain first domesticated by indigenous peoples in southern Mexico about 10,000 years ago. Th ...
'', introgressed
Introgression, also known as introgressive hybridization, in genetics is the transfer of genetic material from one species into the gene pool of another by the repeated backcrossing of an interspecific hybrid with one of its parent species. Intr ...
into B73. The Qgls8 QTL is around 130kilobase
A base pair (bp) is a fundamental unit of double-stranded nucleic acids consisting of two nucleobases bound to each other by hydrogen bonds. They form the building blocks of the DNA double helix and contribute to the folded structure of both DNA ...
in chromosome 8
Chromosome 8 is one of the 23 pairs of chromosomes in humans. People normally have two copies of this chromosome. Chromosome 8 spans about 145 million base pairs (the building material of DNA) and represents between 4.5 and 5.0% of the total DNA ...
.[
]
Crop rotation
The amount of initial inoculum will be reduced when a crop other than corn is planted for ≥2 years in that given area; meanwhile proper tillage methods are carried out. Clean plowing and 1-year crop rotation in the absence of corn allows for greater reductions of the disease as well. Note that conventional tilling can reduce disease but can lead to greater soil erosion.
Residue management
Burying the debris under the last year's crop will help in reducing the presence of ''Cercospora zeae-maydis'', as the fungal-infected debris can only survive above the soil surface. Again this technique will aid in reducing the primary inoculum, but it will not completely eradicate the disease.
Fungicides
Fungicides, if sprayed early in season before initial damage, can be effective in reducing disease.
Currently there are 5 known fungicides that treat Corn grey leaf spot:
Planting date
The impact of GLS is more severe if plants are affected early in their development. Early planting can help reduce yield losses by ensuring the crop is at a later stage of grain fill when conditions are typically favorable for GLS development.
# Headline EC (active ingredient: pyraclostrobin
Pyraclostrobin is a quinone outside inhibitor ( QI)-type fungicide used in agriculture. Among the QIs, it lies within the strobilurin chemical class.
Use
Pyraclostrobin is used to protect ''Fragaria'', ''Rubus idaeus'', ''Vaccinium corymbosum'', ...
)
# Quilt (active ingredient: azoxystrobin
Azoxystrobin is the ISO common name for an organic compound that is used as a fungicide. It is a broad spectrum systemic active ingredient widely used in agriculture to protect crops from fungal diseases. It was first marketed in 1996 using the b ...
+ propiconazole
Propiconazole is a triazole fungicide, also known as a DMI, or demethylation inhibiting fungicide due to its binding with and inhibiting the 14-alpha demethylase enzyme from demethylating a precursor to ergosterol. Without this demethylation st ...
)
# Proline 480 SC (active ingredient: prothioconazole)
# Tilt 250 E, Bumper 418 EC (active ingredient: propiconazole)
:::Headline EC
:::Headline is to be applied at . For optimal disease control, begin applications prior to disease development (see §Disease cycle). This fungicide can only be applied a maximum of 2 applications/year. Ground and aerial application are both acceptable.
:::Quilt
:::Quilt is to be applied at . Application of Quilt is to be made upon first appearance of disease, followed by a second application 14 days after, if environmental conditions are favorable for disease development (see §Disease cycle). Upon browning of corn sheaths, Quilt is not to be applied. This fungicide can only be applied a maximum 2 applications/yr. Ground and aerial application are both acceptable.
:::Proline 480 SC
:::Proline 480 SC is to be applied at . This fungicide can only be applied a maximum 1 time/year. Only ground application is acceptable. A 24-hour re-entry time is required (minimum amount of time that must pass between the time a fungicide is applied to an area or crop and the time that people can go into that area without protective clothing and equipment).
:::Tilt 250 and Bumper 418 EC
:::Tilt 250 is to be applied at . Bumper 418 EC is to be applied at . Both fungicides are to be applied when rust pustules first appear. If disease is prevalent after primary application, a second application 14 days later may be necessary. Two weeks later, a third application can be made under severe amount of disease. Ground and aerial application are both acceptable.
When spraying fungicides Quilt and Headline EC at at tassel stage using a tractor-mounted powered sprayer using , average yield was seen to increase. The use of fungicides can be both economically and environmentally costly and should only be applied on susceptible
Susceptibility may refer to:
Physics and engineering
In physics the susceptibility is a quantification for the change of an extensive property under variation of an intensive property. The word may refer to:
* In physics, the susceptibility of a ...
varieties and large-scale corn production. In order to prevent fungal resistance to fungicides, all fungicides are to be used alternatively, switching fungicides with different modes of action. Pyraclostrobin (Headline EC) and azoxystrobin are Quinone outside Inhibitor ( QI) fungicides, whereas propiconazole and prothioconazole are DeMethylation Inhibitors (DMI) fungicides.
Weed control
By removing weeds, above ground airflow to the crop is increased, relative humidity is decreased, and it limits infection at the most susceptible times.[Lipps, P.E, Mills, D.R, Gray Leaf Spot on Corn. The Ohio State University]
Importance
Before 1970, corn grey leaf spot was not prevalent in the United States, however the disease spread during the mid part of the decade throughout low mountain regions of North Carolina, Kentucky, Tennessee, and Virginia. Today, the disease has expanded to Delaware, Illinois, Indiana, Iowa, Maryland, Missouri, Ohio, Pennsylvania and west Tennessee.
Corn grey leaf spot can be an extremely devastating disease as potential yield losses range from . At higher disease levels, even greater losses can result. When a corn plant's ability to store and produce carbohydrates (glucose) in the grain is diminished, yield losses take place. This occurs when ''Cercospera zeae-maydis'' infects foliar tissue and reduces the plant's ability to photosynthesize and produce byproducts of the process (ex. glucose).
References
{{Reflist
Cercospora
Fungal plant pathogens and diseases
Maize diseases
Leaf diseases